To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, numerous transformer and converter topologies are available, some of which are described in the documents referenced above. All require installation of extra equipment and add to the total cost. Preferred methods are those which perform well in practical power systems which incorporate voltage unbalance and preexisting harmonic voltages. Methods whereby three-phase power is converted to nine-phase power before processing, have been described in U.S. Pat. Nos. 5,124,904, 5,619,407, 6,191,968, and 6,335,872. Continual improvements are sought to reduce cost, particularly in systems with lower output current. The invention described here provides for a lower cost, smaller size design.
This invention describes an improvement to an approach first described in U.S. Pat. No. 4,876,634. In this patent, m sets of three-phase AC voltage each displaced by an angle of nominally 60°/m are applied to m three-phase bridge converters that are paralleled to feed a DC load. In this patent, unwanted circulating currents are minimized by interphase transformers incorporated in the DC circuits of the paralleled converters. Interphase transformers designed with appropriate zero sequence impedance can prevent circulating currents, but they are relatively bulky. It is desirable to eliminate them. This is possible using the methods taught in the U.S. Pat. No. 5,124,904 patent.
U.S. Pat. No. 6,335,872 uses the means described in U.S. Pat. No. 5,124,904 to eliminate the interphase transformer by using a nine-phase configuration. Also, it draws on the polygon transformer described in U.S. Pat. No. 4,876,634 as a means to generate a nine-phase AC supply. Unfortunately, the full potential of this arrangement is not obtained when the polygon transformer is used to feed loads that draw significant third harmonic current, as in an 18-pulse converter fed with a nine-phase supply. The polygon transformer rating is greatly increased because of third harmonic current that circulates in the transformer coils. The invention herein eliminates that problem and results in a significantly smaller transformer.
U.S. Pat. No. 6,101,113 describes means to retain the desirable 60°/m phase shifting features without using interphase transformers. In this patent, unwanted circulating currents are limited by means of zero sequence blocking transformers included in at least m−1 of the m AC voltage sets.
The invention described here combines zero sequence blocking transformers with the polygon transformer described in U.S. Pat. No. 4,876,634. It results in multipulse converter circuits with substantially increased efficiency, reduced size, and lower cost than previously disclosed methods. Circulating harmonic currents are avoided and transformer size is greatly reduced.
In a practical 18-pulse design, the kVA of the polygon autotransformer is about 17% of the DC load kW. In a 24-pulse configuration the transformer rating is less than 11%.
The invention can also be applied with double-wound transformer configurations, as taught in the U.S. Pat. No. 4,255,784 patent. An additional coil or coils on each phase allow any voltage transformer ratio to be achieved. In this case the transformer rating is comparable to the load.
As noted previously, other auto-wound and double-wound transformer methods are available that can dispense with the interphase transformers or ZSBTs by illustratively using a nominal 120°/m phase shift as taught in U.S. Pat. No. 5,124,904. However, these circuits result in shorter current conduction angles, which increase the rms current in conductors and semiconductors. Further, because AC line reactance is invariably included before the transformer, high frequency harmonic voltages are developed across the transformers. Thus, high performance magnetic steel is often used.
The transformer used in the invention described here is operated with AC line reactors in the output leads. This feature not only increases current conduction, which reduces rms current, but also reduces harmonic voltages appearing across the transformer. Thus, the transformer is better able to use lower quality magnetic material. The net result is a much smaller transformer. For example, in an 18-pulse method the transformer of the invention has only about 40% of the rating of typical 18-pulse converters presently in use.
Because of voltage vector differences, the inherent open circuit DC output voltage of the invention is somewhat higher than that of multipulse systems in which the AC voltage sets are displaced by an angle of 120°/m. However, careful selection of the magnetizing inductance of the zero sequence blocking transformer, in conjunction with a minimum load or other means described in the invention, obviates this problem. To highlight the importance of the zero sequence magnetizing impedance the device in this invention is henceforth called ZSBI (zero sequence blocking inductor)